Deep groundwater in the crystalline basement of the Black Forest region

Two major types of groundwater can be readily distinguished in the Variscian crystalline basement of the Black Forest in S–W Germany. Saline thermal water utilized in spas has its origin in 3–4 km deep reservoirs and developed its composition by 3 component mixing of surface freshwater, saltwater (of ultimately marine origin) and a water–rock reaction component. In contrast to the thermal water, CO₂-rich mineral water, tapped and bottled from many wells in the Black Forest, has low salinities but a TDS distribution similar to that of thermal water. It developed its chemical composition entirely by reaction of CO₂-rich water with the gneissic or granitic aquifer rock matrix. Particularly important is the contribution of various plagioclase dissolution and weathering reactions that may, at some locations, involve precipitation and dissolution of secondary calcite. Sodium/Ca ratios of water and of rock forming plagioclase in the basement rocks suggests that plagioclase weathering is strongly incongruent. Calcium is released to the water, whereas Na remains fixed to the albite feldspar component.The major element composition of 192 water samples used in this study also indicates a clear vertical stratification of the type of water chemistry; Ca–HCO₃ near the surface, Na–Ca–HCO₃–SO₄ at intermediate depth and Na–Ca–Cl at great depth.The mean permeability of Black Forest granite is about K=10⁻⁶ m/s; it is significantly lower in gneisses (gneiss: mean K=5×10⁻⁸ m/s) leading to focused flow through granite. Highly permeable fracture and fault zones, particularly in granite, are utilized by high-TDS saline deep groundwater as ascent channels and flow paths. Although spatially closely associated, the topography driven upwelling system of saline deep water and the near-surface flow system of CO₂-rich mineral waters are hydraulically and chemically unconnected.     

Stability of scapolite in the system Ab-An-NaCl-CaCO3 at 4 kb and 750°C

Scapolite solid solution has been synthesized at 750°C and 4 kbar and is stable relative to plagioclase + calcite + halite over the range of plagioclase compositions from Ab₈₅An₁₅ to Ab₇₀An₃₀, although albite + halite is stable relative to marialite, Na₄Al₃Si₉O₂₄Cl, and anorthite + calcite is stable relative to meionite, Ca₄Al₆Si₆O₂₄CO₃. A chloride-free scapolite, mizzonite, has been synthesized at the approximate composition NaCa₃Al₅Si₇O₂₄CO₃ (Ab. 2An. CaCO₃). In the absence of chloride, a three-phase invariant assemblage, sodic plagioclase (~Ab₆₀An₄₀) + scapolite + calcite is stable relative to plagioclase + calcite over the approximate range of plagioclase composition Ab₆₀An₄₀-Ab₃₅An₆₅ and another three-phase invariant assemblage, calcic plagioclase (~Ab₁₅An₈₅) + scapolite + calcite is stable over the approximate range Ab₃₀An₇₀-An₁₅An₈₅.Unit-cell dimensions and refractive indices have been determined for the scapotite synthesized in these experiments and are compared with values for chemically analyzed natural scapolites.Scapolite must be regarded as a ternary solid solution in which, at a given equivalent An-content, the Cl/CO₃ ratio in the large anion site can vary as a function of NaCl and CaCO₃ activities.     

Stability and phase equilibria of chloride and carbonate bearing scapolites at 750°C and 4000 bar

At 750°C and 4000 bar scapolite is stable relative to plagioclase + calcite over the range of plagioclase compositions An₅₃–An₈₃. The assemblage plagioclase + scapolite + calcite is stable relative to plagioclase + calcite over the ranges of plagioclase composition An₄₈-An₅₃ and An₈₃–An_91.5. When NaCl is present in the coexisting fluid the range of scapolite compositions stable relative to plagioclase increases. High mole fractions of NaCl in the fluid stabilize scapolite relative to plagioclases from An₂₅ to An₈₇ in the presence of excess calcite. Determination of the $\text{Cl}\text{(Cl + CO}{}_3\text{)}$ ratios of the synthetic scapolites shows that the range of stable scapolite compositions is significantly larger than heretofore proposed, and that even the chloride and carbonate bearing scapolites must be considered a four component solid solution. The K_D for the exchange of NaCl and CaCo₃ between coexisting scapolite, fluid and carbonate is given by the equation In $\text{K}{}_{\mathrm{D}}\text{= (?0.0028) [}\text{Al}\text{(Al + Si)}\text{]}{}^{\mathrm{?5.5580}}$. This equation implies that Cl-poor natural scapolites coexisted with fluids low in NaCl, and that regional occurrences of Cl-rich scapolites are likely to represent metamorphosed evaporite sequences.     

Mineralkogical and Petrochemical Characterisitics and Genesis of Laoniugou Geneiss in Jiapigou Gold Mine,Jilin Province

Detailed mineralogical and petrochemical studies show that the Laoniugou gneiss of the Jiapigou gold mine is composed mainly of plagioclase gneiss and irregular to lentiform plagioclase amphibolite melanic enclaves.The major element contents show an obvious bimodal and trondhjemitic series evolutional trend.This situation is significantly different from that encountered in bimodal calc-alkalic volcanic rocks in the rift-type Archaean greenstone belt.The contents of Rb,Sr and Ba are 7-21 ppm,153-363ppm and 201-1451 ppm respectively ,close to those of common Archaean grey gneisses.All the samples of plagioclase gneisses show positive Eu anomalies (even up to 4.6).The protoliths of the plagioclase gneiss are high-Al2O3 trondhjemitic series rocks,belonging to typical TTG of Archaean high-grade metamorphic terrain .The gneiss is quite similar to the B-type Amitsoq gneiss of W.Greenland .The authors believe that the plagioclase amphibolite enclaves are the relics of ancient oceanic crust while the plagioclase gneiss is the TTG ancient intrusive rock resulting from partial melting of the oceanic crust.     

Rocks control the chemical composition of surface water from the high Alpine Zermatt area (Swiss Alps)

The study presents composition data of 87 surface water samples from high alpine catchments of the Zermatt area (Swiss Alps). The investigated area covers 170 km². It was found that the surface runoff acquires the dissolved solids mostly by reaction of precipitation water with the minerals of the bedrock. Total dissolved solids (TDS) vary from 6 to 268 mg L^?1. All collected water shows a clear chemical signature of the bedrock mineralogy. The contribution of atmospheric input is restricted to small amounts of ammonium nitrate and sodium chloride. NH₄ is a transient component and has not been detected in the runoff. Evaporation is not a significant mechanism for TDS increase in the Zermatt area. The chemical composition of the three main types of water can be related to the mineralogy of the dominant bedrock in the catchments. Specifically, Ca-HCO₃ (CC) waters develop from metamorphic mafic rocks and from carbonate-bearing schists. Mg-HCO₃ water originates from serpentinites and peridotites. Ca-SO₄ (CS) waters derive from continental basement rocks such as pyrite-rich granite and gneiss containing oligoclase or andesine. The collected data suggest that, together with reaction time, modal sulfide primarily controls and limits TDS of the waters by providing sulfuric acid for calcite (CC waters) and silicate (CS waters) dissolution. If calcite is present in the bedrock, its dissolution neutralizes the acid produced by sulphide weathering and buffers pH to near neutral to weakly alkaline conditions. If calcite is absent, the process produces low-pH waters in gneiss and granite catchments. The type of bedrock and its mineral assemblage can be recognized in water leaving very small catchments of some km² area. The large variety of water with a characteristic chemical signature is an impressive consequence of the richly diverse geology and the different rock inventory of the local catchments in the Zermatt area.     

Ultra-high-pressure metamorphism of carbonate rocks in the Dabie Mountains, central China

Abstract Widespread ultra-high-P assemblages including coesite, quartz pseudomorphs after coesite, aragonite, and calcite pseudomorphs after aragonite in marble, gneiss and phengite schist are present in the Dabie Mountains eclogite terrane. These assemblages indicate that the ultra-high-P metamorphic event occurred on a regional scale during Triassic collision between the Sino-Korean and Yangtze cratons. Marble in the Dabie Mountains is interlayered with coesite-bearing eclogite and gneiss and as blocks of various size within gneiss. Discontinuous boudins of eclogite occur within marble layers. Marble contains an ultra-high-P assemblage of calcite/aragonite, dolomite, clinopyroxene, garnet, phengite, epidote, rutile and quartz/coesite. Coesite, quartz pseudomorphs after coesite, aragonite and calcite pseudomorphs after aragonite occur as fine-grained inclusions in garnet and omphacite. Phengites contain about 3.6 Si atoms per formula unit (based on 11 oxygens). Similar to the coesite-bearing eclogite, marble exhibits retrograde recrystallization under amphibolite–greenschist facies conditions generated during uplift of the ultra-high-P metamorphic terrane. Retrograde minerals are fine grained and replace coarse-grained peak metamorphic phases. The most typical replacements are: symplectic pargasitic hornblende + epidote after garnet, diopside + plagioclase (An₁₈) after omphacite, and fibrous phlogopite after phengite. Ferroan pargasite + plagioclase, and actinolite formed along grain boundaries between garnet and calcite, and calcite and quartz, respectively. The estimated peak P–T conditions for marble are comparable to those for eclogite: garnet–clinopyroxene geothermometry yields temperatures of 630–760°C; the garnet–phengite thermometer gives somewhat lower temperatures. The minimum pressure of peak metamorphism is 27 kbar based on the occurrence of coesite. Such estimates of ultra-high-P conditions are consistent with the coexistence of grossular-rich garnet + rutile, and the high jadeite content of omphacite in marble. The fluid for the peak metamorphism was calculated to have a very low X_CO2 (<0.03). The P–T conditions for retrograde metamorphism were estimated to be 475–550°C at <7 kbar.     

Dissolution kinetics of plagioclase in the melt of the system diopside-albite-anorthite,and origin of dusty plagioclase in andesites

The textures and kinetics of reaction between plagioclase and melts have been investigated experimentally, and origin of dusty plagioclase in andesites has been discussed. In the experiments plagioclase of different compositions (An₉₆, An₆₁, An₅₄, An₂₃, and An₂₂) surrounded by glasses of six different compositions in the system diopside-albite-anorthite was heated at temperatures ranging from 1,200 to 1,410° C for 30 min to 88 h. Textures were closely related to temperature and chemical compositions. A crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt (glass) and remained its original euhedral shape below the liquidus. Whatever the temperature, the crystal-melt interface became rough and often more complicated (sieve-like texture composed of plagioclase-melt mixture in the scale of a few m was developed from the surface of the crystal inward; formation of mantled plagioclase) if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt, and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase. From these results the following two types of dissolution have been recognized; (1) a crystal simply dissolves in the melt which is undersaturated with respect to the phase (simple dissolution), and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt (partial dissolution). The amount of a crystal dissolved and reacted increased proportional to the square root of time. This suggests that these processes are controlled by diffusion, probably in the crystal.Mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so-called resorbed plagioclase in igneous rocks. Chemical compositions and textures of plagioclase phenocrysts in island-arc andesites of magma mixing origin have been examined. Cores of clear and dusty plagioclase were clacic (about An₉₀) and sodic (about An₅₀), respectively. This result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing.     

Grain-size-sensitive creep of plagioclase accompanied by solution–precipitation and mass transfer under mid-crustal conditions

We report here on a study of three deformed granitoids: two mylonites and an ultramylonite from the inner ductile shear zone of the Ryoke metamorphic belt, SW Japan. Monophase layers composed of quartz, plagioclase or K-feldspar are present in all samples. The plagioclase-rich layers consist of grains 6–10 μm in size, and sometimes include patchy K-feldspar and quartz, indicating solution-precipitation. In the mylonite, the fine-grained plagioclase is mainly An_23–25 and, the composition of plagioclase porphyroclast is An_21–39 without any significant maximum. The An compositions together with textural observations indicate that fine-grained plagioclase nucleated from solution with mass transfer during deformation. In the ultramylonite, fine-grained plagioclase is widely changed to be An_15–37, indicating that the grain-size-reduction process includes fracturing of original plagioclase porphyroclasts in addition to the solution–precipitation process, which results in the composition concentrated around An₃₀. In all samples, the crystallographic orientations of fine-grained plagioclases are almost random and do not correlate with neighbouring porphyroclasts. Grain-size-sensitive creep occurred during rock deformation subsequent to the process of solution–precipitation that involved mass transfer via fluids.     

The origin of plagioclase phenocrysts in basalts from continental monogenetic volcanoes of the Kaikohe-Bay of Islands field,New Zealand: implications for magmatic assembly and ascent

Late Quaternary, porphyritic basalts erupted in the Kaikohe-Bay of Islands area, New Zealand, provide an opportunity to explore the crystallization and ascent history of small volume magmas in an intra-continental monogenetic volcano field. The plagioclase phenocrysts represent a diverse crystal cargo. Most of the crystals have a rim growth that is compositionally similar to groundmass plagioclase (~?An₆₅) and is in equilibrium with the host basalt rock. The rims surround a resorbed core that is either less calcic (~?An_20–45) or more calcic (>?An₇₀), having crystallized in more differentiated or more primitive melts, respectively. The relic cores, particularly those that are less calcic (<?~?An₄₅), have ⁸⁷Sr/⁸⁶Sr ratios that are either mantle-like (~?0.7030) or crustal-like (~?0.7040 to 0.7060), indicating some are antecrysts formed in melts fractionated from plutonic basaltic forerunners, while others are true xenocrysts from greywacke basement and/or Miocene arc volcanics. It is envisaged that intrusive basaltic forerunners produced a zone where various degrees of crustal assimilation and fractional crystallization occurred. The erupted basalts represent mafic recharge of this system, as indicated by the final crystal rim growths around the entrained antecrystic and xenocrystic cargo. The recharge also entrained cognate gabbros that occur as inclusions, and produced mingled groundmasses. Multi-stage magmatic ascent and interaction is indicated, and is consistent with the presence of a partial melt body in the lower crust detected by geophysical methods. This crystallization history contrasts with traditional concepts of low-flux basaltic systems where rapid ascent from the mantle is inferred. From a hazards perspective, the magmatic system inferred here increases the likelihood of detecting eruption precursor phenomena such as seismicity, degassing and surface deformation.     

Dynamics of magma mixing and degassing recorded in plagioclase at Stromboli (Aeolian Archipelago,Italy)

Crystal-rich materials (scoriae and lava flows) emitted during the 1985–2000 activity of Stromboli were taken into consideration for systematic study of bulk rock/matrix glass chemistry and in particular for the study of chemical and textural zoning of plagioclase, the most abundant mineral phase. Over the considered time period, bulk rock composition remained fairly constant in both major (SiO₂ 49.2–50.9 wt% and K₂O 1.96–2.18 wt%) and trace elements. The quite constant chemistry of matrix glasses also indicates that the degree of crystallization of magma was maintained at around 50 vol%. Plagioclase ranges in composition between An₆₂ and An₈₈ and is characterized by alternating, <10–100 m thick, bytownitic and labradoritic concentric layers, although the dominant and representative plagioclase of scoriae is An_68. The labradoritic layers (An_62–70) show small-scale (1–5 µm), oscillatory zoning, are free of inclusions, and appear to record episodes of slow crystal growth in equilibrium with a degassed liquid having the composition of the matrix glass. In contrast the bytownitic layers (An₇₀-An₈₈) are patchy zoned, show sieve structure with abundant micrometric glass inclusions and voids, and are attributed to rapid crystal growth.A key to understanding the origin of bytownitic layers can be retrieved from the texture and composition of the coronas of plagioclase xenocrysts, inherited from crystal-rich magma, in nearly aphyric pumice which are erupted during more energetic events and represent a deep, volatile-rich, HK-basaltic magma. They show a continuum from fine-sieve to evident skeletal texture from the inner to the outer part of the corona associated with normal compositional zoning from An₉₀ to An₇₅. In the light of these observations, we propose that input of H₂O-rich melt blobs, and their mixing with the residing magma, causes partial dissolution of the labradoritic layers followed by the growth of bytownitic composition whose sieve texture attests of rapid crystallization occurring under undercooling conditions mainly induced by degassing. As a whole, the zoning of plagioclase in the scoriae records successive and discrete intrusions of volatile-rich magma blobs, its degassing and mixing with the resident degassed magma at shallow level.Editorial responsibility: T.L. GroveAn erratum to this article can be found at     

Trapped intercumulus liquid in the Main Zone of the eastern Bushveld Complex, South Africa

Lateral variations in the amount of trapped intercumulus liquid in the Main Zone of the eastern Bushveld Complex are constrained by new Sr-isotopic, whole-rock and mineralogical data from three profiles that are separated laterally by ca.100 km and represent thicknesses of 551–1,127 m of Main Zone gabbronorites below the Pyroxenite Marker. An analysis of the An-contents (100×Ca/(Ca+Na)) of plagioclase cores within the Thornhill (north), Roossenekal and Stoffberg (south) profiles show similar systematic, up-section variations from An₆₆ to An₅₉. In contrast, both the An-contents of bulk plagioclase separates and the Mg-numbers (100×Mg/(Mg+Fe_T)) of orthopyroxenes show pronounced lateral variations from Thornhill (An_67-61; Mg#_67-61), through Roossenekal (An_64-58; Mg#_64-55) to Stoffberg (An_59-55; Mg#_59-50). These mineralogical variations are interpreted to be the result of reaction between cumulus minerals and an increasing amount of trapped liquid from north to south. Modelling of the trapped liquid shift of orthopyroxene compositions suggests that the amount of trapped liquid in the cumulates increased from near 0% at Thornhill, through 10–30% at Roossenekal to 30–45% at Stoffberg. A two- to eightfold southward increase in whole-rock concentrations of P, Ti, Y, Zr and Ba is consistent with the trapped liquid model. However, the 14-fold increase in Rb from Thornhill to Stoffberg is too great to be accounted for by trapped liquid alone, but can be explained by local assimilation of partial melts of the country rock. Constant initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of Main Zone plagioclase separates (0.7081–0.7085) in all the three profiles do not preclude assimilation of adjacent basaltic to rhyolitic country rock with initial ratios between 0.6924–0.7096. The southward increase in the amount of trapped liquid is ascribed to an increased cooling rate by enhanced heat loss and partial assimilation of country rock xenoliths in the distal cumulate sequence at Stoffberg. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorised users.     

Irreversible water–rock mass transfer accompanying the generation of the neutral,Mg–HCO3 and high-pH,Ca–OH spring waters of the Genova province,Italy

In a recent survey of the spring waters of the Genova province, many neutral Mg–HCO₃ waters and some high-pH, Ca–OH waters were found in association with serpentinites. All the springs are of meteoric origin as indicated by the stable isotopes of water and dissolved N₂ and Ar. Interaction of these meteoric waters with serpentinites determines a progressive evolution in the chemistry of the aqueous phase from an immature Mg-rich, SO₄–Cl facies of low salinity to an intermediate Mg–HCO₃ facies (pH 7.0–8.5, P_CO210^−3.5–10^−2.5 bar, Eh 150–250 mV), and to a mature Ca–OH facies (pH 10–12, P_CO2 10^−9.4−10^−10.6 bar, Eh-390 to-516 mV). The irreversible water–rock mass transfer leading to these chemical changes in the aqueous phase was simulated through reaction path modeling, assuming bulk dissolution of a local serpentinite, and the precipitation of gibbsite, goethite, calcite, hydromagnesite, kaolinite, a montmorillonite solid mixture, a saponite solid mixture, sepiolite, and serpentine. The simulation was carried out in two steps, under open-system and closed-system conditions with respect to CO₂, respectively. The calculated concentrations agree with analytical data, indicating that the computed water-rock mass transfer is a realistic simulation of the natural process. Moreover, the simulation elucidates the role of calcite precipitation during closed-system serpentinite dissolution in depleting the aqueous solution of C species, allowing the concurrent increment in Ca and the acquisition of a Ca–OH composition. Calcium–OH waters, due to their high pH, tend to absorb CO₂, precipitating calcite. Therefore, these waters might be used to sequester anthropogenic CO₂, locally preventing environmental impact to the atmosphere.     

Mass transfer and porosity evolution during low temperature water–rock interaction in gneisses of the simano nappe: Arvigo,Val Calanca,Swiss Alps

Late Alpine fissures and fractures in amphibolite-facies basement gneisses at Arvigo (Val Calanca, Swiss Alps) show distinct cm-sized reaction selvages parallel to the fracture walls that composed of subgreenschist facies assemblages produced by the interaction of water present in the fracture porosity with the old high-grade gneiss assemblages. The process of selvage or reaction-vein formation occurred in the brittle deformation regime and at temperatures characteristic of, first the prehnite-pumpellyite facies and then later of the zeolite facies. The vein formation occurred during uplift and cooling at very late stages of the Alpine orogeny. The reaction veins are composed of a selvage of altered gneiss on both sides of the central fracture and a central zone with fissure minerals that have been growing in the open fracture pore space. The central zone of the Arvigo veins contains an early assemblage with epidote, prehnite and chlorite and a late succession sequence of various species of zeolite. The veins of the Arvigo quarry are convincing evidence that fracture fluids in gneiss and granite have the potential to precipitate Ca–zeolite. This is an important find because many fluids recovered from deep continental drill holes and from geothermal energy exploration are found to be oversaturated in respect to a number of Ca–zeolite species. Vein formation during late uplift and cooling of the Alps occurred at continuously decreasing T and at hydrostatic pressure: (1) coexisting prehnite/epidote records temperatures of 330–380°C, (2) chlorite formation at temperature of 333 ± 32°C and (3) formation of zeolites <250°C. In the selvages the prime reaction is the replacement of plagioclase by albite along a sharp reaction front that separates the selvage from unaltered gneiss. In addition to albitisation, chloritisation of biotite is the second important reaction in the alteration process. The reactions release components for the formation of Ca–Al silicates. The water–rock interaction is associated with a depletion of Al, Si, Ca, Fe and K in the altered wall rock. The overall reaction is associated with an increase in porosity of up to 14.2 ± 2.2% in the selvage zone (altered wall rock), caused by the volume decrease during albitisation and the removal of biotite. The propagation of the sharp reaction front through the gneiss matrix occurred via a dissolution-reprecipitation mechanism. Zeolite formation is tied to the plagioclase alteration reaction in the rock matrix, which releases components for zeolite formation to a CO₂-poor aqueous liquid.     

Diffusion controlled growth of pyroxene-bearing margins on amphibolite bands in the granulite facies of Rogaland (Southwestern Norway): implications for granulite formation

In the Rogaland granulites of Southern Norway, thin anhydrous pyroxene-bearing margins (5–10 mm) are observed mainly in migmatitic banded gneisses at the contact between hornblende-rich metabasites and charnockites. According to field data, the development of these margins post-dates any deformation. Petrographic data show that they are zoned. Three different types have been recognized:

1. Metabasite/plagioclase + orthopyroxene/plagioclase + clinopyroxene/gneiss
2. Metabasite/plagioclase + orthopyroxene + clinopyroxene/plagioclase + clinopyroxene/gneiss
3. Metabasite/plagioclase + orthopyroxene/gneiss

The first zone corresponds to the reaction of amphibole and biotite of the metabasite into pyroxenes. The second zone, which is not present in the third type, developed essentially at the expense of gneiss and the Z1/Z2 boundary is likely to correspond to the original contact between metabasite and gneiss. When the anorthite content of plagioclase and the Fe no. of pyroxenes are strongly different between adjacent metabasite (An_38–40 for plagioclase; Fe no. [Fe/(Fe + Mg)], 0.51–0.52 for orthopyroxene) and gneiss (An_22–26; Fe no., 0.58–0.59), the solid solution compositions evolve continuously through the margin from the value in the metabasite to that in the gneiss. On the other hand, a margin is also present when plagioclase and pyroxenes have similar compositions in the adjacent rocks implying that reactions can also take place in the absence of contrasted mineral compositions. The continuous change in solid solution composition as well as evidence of transport in both directions indicates diffusion rather than infiltration as the dominant metasomatic mechanism. The small thickness of the margins is also more typical of a diffusion process. Isocon diagrams demonstrate that Al, Ti, and P are perfectly inert components and that no significant mass or volume change occurred during the margin development. Mass balance of this small-scale granulite formation has been estimated with reference to a perfectly inert component (Ti) and assuming that the metabasite bands were compositionally homogeneous. Most of the geochemical variation is mineralogically controlled. Relative to metabasite, Si and Na are increased due to pyroxenes crystallization and to compositional change of plagioclase when it occurs. Potassium decreases because of biotite disappearance. There is no significant variation in U content but Th is slightly decreased. Zirconium and Hf are not affected whereas Nb, Ta and Mn display the largest increase that requires the involvement of a larger volume of metabasite than that observed in the margin. The K/Rb ratio is increased. Fluorine is strongly depleted due to destabilization of amphibole and biotile. The rare-earth element content in margins is either similar to that of the metabasite or intermediate between that of gneiss and metabasite. This last feature is induced by the development of margins at the expense of both adjacent rock types. Saturation surfaces in chemical potential space provide a graphical method for determination of the parameters controlling the diffusion process. In the simplified system CaO?MgO?SiO₂?Al₂O₃?H₂O, these chemical potential diagrams show that evolution along a $(\mu )_{H_2 O} $ gradient cannot take into account the three different types of margins. A $(\mu )_{H_2 O} $ gradient is thus not prerequisite to the margin development. On the other hand, the succession of zones observed in the different types of margins can be obtained in a $(\mu )_{CaO} - (\mu )_{MgO} - (\mu )_{SiO_2 } $ diagram. This suggests that the $(\mu )_{SiO_2 } $ gradient existing between the two adjacent rocks controls the margin development in all cases. Moreover, the variable contrast of plagioclase composition between the adjacent rocks is responsible for the presence of one or two pyroxenes is the first zone. The absence of the second clinopyroxene-bearing zone in the third type of margin is likely due to the scarcity of orthopyroxene in the gneiss.     

Composition,physical properties and origin of sodic anorthites from the ejected plutonic blocks of the Soufrière Volcano,St. Vincent,West Indies

Three complete analyses and one partial analysis are recorded of unzoned sodic anorthites having compositions An_94,9, An_94,0, An_92,7 and An₉₂ (mol per cent). Although crystallising at a high temperature the K and Sr contents are low. X-ray parameters show the material to be in a relatively high structural state comparable with material from Japanese volcanics. Available data indicate small but real differences in optical parameters between sodic anorthites of volcanic and plutonic origin but this needs further clarification. Refractive indices and specific gravities of the analysed samples are also recorded. Anorthite (An₉₀-An₁₀₀) occurs widely as phenocrysts in basic lavas and as crystal ejecta in the calc-alkaline rock suites of the circumoceanic islands. Anorthite can be precipitated from sub-alkaline magmas under certain conditions and does not require the postulation of special magmas or of contamination. It is apparent that factors other than the normative feldspar composition of the magma determine the composition of the plagioclase precipitated. Evidence from experimental work on synthetic systems indicates that an increase in both hydrostatic pressure and water vapour pressure will influence the plagioclase composition but it is not possible to evaluate these effects fully at the present time. The occurrence of anorthite is interpreted as evidence of extensive crystal accumulation at depth from fractionating basic magmas in circumoceanic calc-alkaline suites.     

Growth of plagioclase rims around metastable kyanite during decompression of high‐pressure felsic granulites (Bohemian Massif)

Samples of high‐pressure felsic granulites from the Bohemian Massif (Variscan belt of Central Europe) characterized by a peak metamorphic (high‐pressure) mineral assemblage of garnet – kyanite – plagioclase – K‐feldspar – quartz ± biotite show well‐developed plagioclase reaction rims around kyanite grains in two microstructural settings. In one setting, kyanite is randomly distributed in the polyphase matrix, whereas in the other setting, it is enclosed within large perthitic K‐feldspar. Kyanite is regarded as a relict of the high‐pressure metamorphic assemblage that became metastable during transition to a low‐pressure overprint. Plagioclase rims from both microstructural settings show continuous outwards decrease of the anorthite content from An_32–25 at the contact with kyanite to An_20–19 at the contact with the matrix or to the perthitic K‐feldspar respectively. Based on mass balance considerations, it is shown that in some cases, a small amount of kyanite was consumed in the rim‐forming reaction to provide the Al₂O₃ component for the growth of plagioclase, whereas in other cases no Al₂O₃ from kyanite was necessary. In a majority of examples, the necessary Al₂O₃ was supplied with CaO and Na₂O from the surrounding matrix material. For kyanite in perthite, a thermodynamic analysis reveals that the kyanite became metastable at the interface with the host perthite at the peak metamorphic pressure, and therefore the plagioclase rim started to grow at ～ 18 kbar. In contrast, kyanite in the polyphase matrix remained stable down to pressures of ～ 16 kbar, and the plagioclase rim only started to grow at a later stage during the decompression. Plagioclase rims around kyanite inclusions within large perthite have a radial thickness of up to 50 μm. In contrast, the radial thickness of plagioclase rims around kyanite in the polycrystalline matrix is significantly larger, up to 200 μm. Another peculiarity is that the plagioclase rims around kyanite in the matrix are polycrystalline, whereas the plagioclase rims around kyanite inclusions in perthitic hosts are single crystals with the same crystallographic orientation as the host perthite. The difference in rim thickness for the two microstructural settings is ascribed to the differences in the efficiency of chemical mass transfer next to the reaction site. The comparatively large thickness of the plagioclase rims grown around kyanite in the matrix is probably due to efficient material transport along the grain and phase boundaries in the matrix. In contrast, chemical mass transfer was comparatively slow in the large perthitic K‐feldspar grains.     

High‐grade localized metasomatic alteration of the granitic gneiss surrounding a clinopyroxene‐rich pegmatoid dyke: Söndrum stenhuggeriet,Halmstad, SW Sweden

The Söndrum stone quarry (Halmstad, SW Sweden) exposes a transition from migmatized granitic gneissic country rock into a foliated clinopyroxene‐free granitic gneiss, and then a central pegmatoid dyke dominated by clinopyroxene megacrysts. This transition zone represents a fracture‐controlled, fluid‐alteration zone that developed under conditions of 650–700 °C and 790 MPa. Mineral chemical trends in F, Cl, Fe, Ti, Mn and Y are interpreted as documenting a fluid infiltration event associated with the formation of the pegmatoid dyke. Fluid inclusions from the pegmatoid dyke are CO₂ dominant, whereas in the surrounding country rock they are dominated by H₂O‐NaCl‐CaCl₂. Fluid inclusions from the intermediate foliated clinopyroxene‐free granitic gneiss are a mixture of the two types. The pegmatoid dyke appears to have originated from a high Ca activity, Fe‐Mg enriched, fluid‐rich granitic melt with a CO₂ component, which was emplaced along a tectonic fracture in a regionally migmatized granitic gneiss in the lower crust. This was accompanied by limited partial melting of the surrounding granitic gneiss. The Ca activity of the melt was high enough to allow for the formation of clinopyroxene megacrysts as opposed to orthopyroxene. H₂O‐enriched fluids expelled from the crystallizing pegmatoid dyke, which retained the majority of the CO₂ helping to stabilize the clinopyroxene. The expelled fluids coarsened and chemically affected the surrounding country rock resulting in trends in the mineral and fluid inclusion chemistry seen today in the foliated and regional granitic gneiss.     

Climatic and lithologic controls on the temporal and spatial variability of CO2 consumption via chemical weathering: An example from the Australian Victorian Alps

A detailed geochemical study on river waters of the Australian Victorian Alps was carried out to determine: (i) the relative significance of silicate, carbonate, evaporite and sulfide weathering in controlling the major ion composition and; (ii) the factors regulating seasonal and spatial variations of CO₂ consumption via silicate weathering in the catchments. Major ion chemistry implies that solutes are largely derived from evaporation of precipitation and chemical weathering of carbonate and silicate lithologies. The input of solutes from rock weathering was determined by calculating the contribution of halite dissolution and atmospheric inputs using local rain and snow samples. Despite the lack of carbonate outcrops in the study area and waters being undersaturated with respect to calcite, the dissolution of vein calcite accounts for up to 67% of the total dissolved cations, generating up to 90% of dissolved Ca and 97% of Mg. Dissolved sulfate has δ³⁴S values of 16 to 20‰_CDT, indicating that it is derived predominantly from atmospheric deposition and minor gypsum weathering and not from bacterial reduction of FeS₂. This militates against sulphuric acid weathering in Victorian rivers. Ratios of Si vs. the atmospheric corrected Na and K concentrations range from ~ 1.1 to ~ 4.3, suggesting incongruent weathering from plagioclase to smectite, kaolinite and gibbsite.Estimated long-term average CO₂ fluxes from silicate weathering range from ~ 0.012 × 10⁶ to 0.039 × 10⁶ mol/km²/yr with the highest values in rivers draining the basement outcrops rather than sedimentary rocks. This is about one order of magnitude below the global average which is due to low relief, and the arid climate in that region. Time series measurements show that exposure to lithology, high physical erosion and long water–rock contact times dominate CO₂ consumption fluxes via silicate weathering, while variations in water temperature are not overriding parameters controlling chemical weathering. Because the atmospheric corrected concentrations of Na, K and Mg act non-conservative in Victorian rivers the parameterizations of weathering processes, and net CO₂ consumption rates in particular, based on major ion abundances, should be treated with skepticism.     

Petrology, Mineralogy and Geochemisty of Antarctic Mesosiderite GRV 020175: Implications for Its Complex Formation History

<正>GRV 020175 is an Antarctic mesosiderite,containing about 43 vol%silicates and 57 vol% metal.Metal occurs in a variety of textures from irregular large masses,to veins penetrating silicates, and to matrix fine grains.The metallic portion contains kamacite,troilite and minor taenite.Terrestrial weathering is evident as partial replacement of the metal and troilite veins by Fe oxides.Silicate phases exhibit a porphyritic texture with pyroxene,plagioclase,minor silica and rare olivine phenocrysts embedded in a fine-grained groundmass.The matrix is ophitic and consists mainly of pyroxene and plagioclase grains.Some orthopyroxene phenocrysts occur as euhedral crystals with chemical zoning from a magnesian core to a ferroan overgrowth;others are characterized by many fine inclusions of plagioclase composition.Pigeonite has almost inverted to its orthopyroxene host with augite lamellae, enclosed by more magnesian rims.Olivine occurs as subhedral crystals,surrounded by a necklace of tiny chromite grains(about 2-3μm).Plagioclase has a heterogeneous composition without zoning. Pyroxene geothermometry of GRV 020175 gives a peak metamorphic temperature(～1000℃) and a closure temperature(～875℃).Molar Fe/Mn ratios(19-32) of pyroxenes are consistent with mesosiderite pyroxenes(16-35) and most plagioclase compositions(An_(87.5_96.6)) are within the range of mesosiderite plagioclase grains(An_(88-95)).Olivine composition(Fo_(53.8)) is only slightly lower than the range of olivine compositions in mesosiderites(Fo_(55-90)).All petrographic characteristics and chemical compositions of GRV 020175 are consistent with those of mesosiderite and based on its matrix texture and relatively abundant plagioclase,it can be further classified as a type 3A mesosiderite.Mineralogical, penological,and geochemical studies of GRV 020175 imply a complex formation history starting as rapid crystallization from a magma in a lava flow on the surface or as a shallow intrusion.Following primary igneous crystallization,the silicate underwent varying degrees of reheating.It was reheated to 1000℃,followed by rapid cooling to 875℃.Subsequently,metal mixed with silicate,during or after which,reduction of silicates occurred;the reducing agent is likely to have been sulfur.After redox reaction,the sample underwent thermal metamorphism,which produced the corona on the olivine, rims on the inverted pigeonite phenocrysts and overgrowths on the orthopyroxene phenocrysts,and homogenized matrix pyroxenes.Nevertheless,metamorphism was not extensive enough to completely reequilibrate the GRV 020175 materials.     

An electron‐optical study of melt‐related microstructures in granulite facies rocks from the Torngat Orogen

Cordierite–quartz and plagioclase–quartz intergrowths in a paragneiss from northern Labrador (the Tasiuyak Gneiss) were studied using SEM, STEM and TEM. The gneiss experienced granulite facies conditions and partial melting during both regional and, subsequently, during contact metamorphism. The microstructures examined all results from the contact metamorphism. Cordierite–quartz intergrowths occur on coarse and fine scales. The former sometimes exist as a ‘geometric’ intergrowth in which the interface between cordierite and quartz appears planar at the resolution of the optical microscope and SEM. The latter exists in several microstructural variants. Plagioclase is present as a minor component of the intergrowth in some examples of both the coarse and fine intergrowth. Grain boundaries in cordierite–quartz intergrowths are occupied by amorphous material or a mixture of amorphous material and chlorite. Cordierite and quartz are terminated by crystal faces in contact with amorphous material. Chlorite is sometimes found on cordierite surfaces and penetrating into cordierite grains along defects. Quartz contains (former) fluid inclusions 10–20 nm in maximum dimension. The presence of planar interfaces between cordierite and the amorphous phase is reminiscent of those between crystals and glass in volcanic rocks, but in the absence of compelling evidence that the amorphous material represents former melt, it is interpreted as a reaction product of cordierite. Plagioclase–quartz intergrowths occur in a number of microstructural variants and are commonly associated with cordierite–quartz intergrowths. The plagioclase–quartz intergrowths display simple, non‐planar interfaces between plagioclase and quartz. Quartz contains (former) fluid inclusions of dimensions similar to those observed in cordierite–quartz intergrowths. The boundary between quartz and enclosing K‐feldspar is cuspate, with quartz cusps penetrating a few tens of nanometres into K‐feldspar, commonly along defects in K‐feldspar and sometimes with very low dihedral angles at their tips. This cuspate microstructure is interpreted as melt pseudomorphs. The plagioclase–quartz intergrowths share some features with myrmekite, but differ in some respects: the composition of the plagioclase (An₃₇Ab₆₂Or₁–An₃₈Ab₆₁Or₁); the association with cordierite–quartz intergrowths; and microstructures that are atypical of myrmekite (e.g. quartz vermicules shared with cordierite–quartz intergrowths). It is inferred that the plagioclase–quartz intergrowths may have formed from, or in the presence of, melt. Inferred melt‐related microstructures preserved on the nanometre scale suggest that melt on grain boundaries was more pervasive than is evident from light optical and SEM observations.